Electrically Conductive Joining Agent and Solder Joint

ABSTRACT

This invention has an object to provide an electrically conductive joining agent which enables a thermosetting resin to cure in a short time. It contains electrically conductive metallic powder including Sn of 40% or more, thermosetting resin, an acid-anhydride-based hardening agent and an organic acid. The electrically conductive metallic powder and the organic acid are reacted during a heating process to produce an organic acid metal salt which is used as hardening accelerator. It enables thermosetting resin to cure in a short time, for example, a time equivalent to a time that is required for the general reflow process.

TECHNICAL FIELD

The present invention relates to an electrically conductive joining agent that contains thermosetting resin using an organic acid metal salt as a hardening accelerator, and a solder joint.

BACKGROUND TECHNOLOGY

Electrically conductive bonding material is a general term for an electrically conductive adhesive agent and the electrically conductive joining agent. In the other words, the electrically conductive adhesive agent indicates an agent that is used for allowing the metals to adhere to each other at lower temperature than a melting point of electrically conductive metallic powder. The electrically conductive joining agent indicates an agent that is used for melting the electrically conductive metallic powder and joining the metals to each other.

Solder has been often used as the electrically conductive joining agent for joining various kinds of electronic circuits to a circuit board and the solder has been melted so that the various kinds of electronic circuits are joined to the circuit board. Although lead-free solder such as Sn—Ag—Cu or the like has been used as solder material, taking into consideration any environmental influences, this solder has a higher liquidus temperature than that of the conventional lead-containing solder by 30 degrees C. or higher. Since temperature within a reflow furnace is very higher than the liquidus temperature, this gives a high thermal shock (thermal stress) to the electronic parts and/or the circuit board to be joined.

Parts-binding material using the electrically conductive adhesive agent has been studied with the object of reducing the thermal stress. This electrically conductive adhesive agent contains electrically conductive metallic powder, thermosetting resin, a hardening agent, a hardening accelerator and the other and pieces of the electrically conductive metallic powder are densely contacted to each other following the hardening of the resin by heating so that various kinds of electronic circuits can be joined to the circuit board without melting the electrically conductive metallic powder (see patent documents 1 and 2).

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Patent Application Publication No.     2011-061241 -   Patent Document 2: Japanese Patent Application Publication No.     2012-067274 -   Patent Document 3: Japanese Patent Application Publication No.     2001-219294 -   Patent Document 4: Japanese Patent Application Publication No.     2010-144150

Non Patent Documents

-   Non Patent Document 1: Panasonic Technical Journal, Vol. 59 No. 1,     72-77

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

When an amine-based hardening agent is used as the hardening agent, it is possible to cure the thermosetting resin quickly. On the other side, it is required to be managed under frozen storage of, for example, −10 degrees C. to prevent any curing reaction from occurring.

When an acid anhydride is used as the hardening agent, it takes a long time to cure the resin, which is hard to be used from a view of a manufacture process. In order to shorten the curing time, it is necessary to add any suitable hardening accelerator.

Further, even when melting the solder to join various kinds of electronic circuits to a circuit board, any bonding materials to which thermosetting resin, a hardening agent for curing the thermosetting resin and the like are added have been also proposed (see patent documents 3 and 4). They, however, include the same problem as that of the above-mentioned electrically conductive adhesive agent.

Non patent document 1 discloses that Sn-3.0Ag-0.5Cu solder (each numerical value indicates weight % and in the following solder composition, it also indicates weight % unless otherwise stated), epoxy resin and flux containing glutaric acid are heated to produce any organic acid metal salts, and the organic acid metal salts open an epoxy ring. The curing time therefor, however, requires around 7 hours.

Although Ag has been used as filler in the patent document 2, low Ag content in the solder is in progress because Ag is expensive. It is desirable to adopt any filler instead of Ag even in the electrically conductive adhesive agent.

This invention has solved such problems and has an object to provide an electrically conductive joining agent which cures thermosetting resin using the acid-anhydride-based hardening agent and allows the thermosetting resin to cure in a short time on the assumption of the electrically conductive joining agent in which the electrically conductive metallic powder is melted for allowing the metals to joint to each other, and a solder joint.

Means for Solving the Problems

The inventors of this application have focused on that an organic acid metal salt produced by reaction of the electrically conductive metallic powder and the organic acid during a heating process functions as a hardening accelerator and have found out that the thermosetting resin can be cured thereby in a short time, for example, a time equivalent to a time that is required for the general reflow process.

This invention contains electrically conductive metallic powder including Sn of 40% or more, thermosetting resin, an acid-anhydride-based hardening agent and an organic acid wherein the electrically conductive metallic powder and the organic acid are reacted to produce an organic acid metal salt and the organic acid metal salt is used as hardening accelerator.

It is preferable that as the thermosetting resin, resin having flexibility such as epoxy resin of bisphenol A type to which an aliphatic skeleton is given is used. This resin allows both of flexibility and toughness to be given thereto. It is also preferable that as the hardening agent, acid anhydride is used. It is further preferable to mix the resin having flexibility and the hardening agent so that they are 1:2 by a molar ratio. In addition, it is preferable to add the organic acid of 2 through 8 weight %, more preferably, 4 through 8 weight % to produce the organic acid metal salt.

Effect of the Invention

According to the invention, since the metallic powder and the organic acid are reacted during the heating process to produce the organic acid metal salt and the organic acid metal salt functions as hardening accelerator, it is possible to cure the thermosetting resin in a short time. Further, since the heating allows the hardening accelerator to be produced, any curing does not suddenly progress in the storage thereof. It is possible to perform refrigeration storage thereof at 2 through 10 degrees C., thereby enabling storage stability to be improved.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph showing a temperature profile of executed examples.

FIG. 2 is a graph showing a temperature profile of an executed example.

FIG. 3 is a graph showing a temperature profile of an executed example.

FIG. 4 is a graph showing a temperature profile of an executed example.

EMBODIMENTS FOR CARRYING OUT THE INVENTION Composition Example of Electrically Conductive Joining Agent According to the Embodiment

Electrically conductive joining agent of this invention contains electrically conductive metallic powder including Sn of 40% or more, thermosetting resin, a hardening agent and an organic acid.

As curing resin, there is resin that is cured by heat, light, ultraviolet rays or the like. As the curing resin, epoxy-based resin, phenolic resin, polyimide-based resin, silicon resin, polyurethane resin, unsaturated polyester resin and the like are conceivable, but the thermosetting resin is used because in the resin that is cured by light or ultraviolet rays, it is impossible to cure any resin positioned under the electronic parts when mounting the parts. As the thermosetting resin, the epoxy resin is the optimal resin.

As the epoxy resin, epoxy resin of bisphenol type is selected. As the bisphenol type, bisphenol A type, bisphenol AP type, bisphenol AF type, bisphenol B type, bisphenol BP type, bisphenol C type, bisphenol E type, bisphenol F type, bisphenol G type, bisphenol M type, bisphenol S type, bisphenol P type, bisphenol PH type, bisphenol TMC type, bisphenol Z type and the like are exemplified. Preferably, it is the bisphenol A type.

It is said that the epoxy resin is excellent in electric and/or mechanical joining properties but is fragile and poor in drop impact characteristic. This is because when curing the epoxy resin, any peeling occurs at an electrode interface to raise a crack.

When using any flexible resin in which, for example, an aliphatic skeleton is given to the epoxy resin, both the flexibility and the toughness are reinforced so that it is possible to prevent any crack by surface peeling from occurring.

Hardening agent is used to cure the epoxy resin. As the hardening agent, amine, an acid anhydride and the like can be used. When the amine is used, the reaction is progressed even if the refrigeration storage is performed, so that the epoxy resin is cured. Therefore, the acid anhydride is selected to perform the refrigeration storage thereof.

As the acid anhydride, acetic acid anhydride, propionic anhydride, succinic anhydride, maleic anhydride and the like are exemplified. In this embodiment, the succinic anhydride is used.

It takes a long time until the hardening merely by heating the epoxy resin and the acid anhydride. Therefore, the hardening accelerator is required. As the hardening accelerator, for example, phenolic compound, tertiary amine, quaternary ammonium salt, quaternary phosphonium salt, imidazole, an organic acid metal salt, Lewis acid and the like are exemplified. In this embodiment, the organic acid metal salt is used.

The metallic powder constituting the organic acid metal salt may be replaced by the electrically conductive metallic powder itself for joining which is added to resin for joining. In this case, it is required to separately add nothing in order to produce the organic acid metal salt. Of course, the electrically conductive metallic powder may be separately added to produce the organic acid metal salt. The organic acid metal salt produced by the reaction of the metallic powder and the organic acid during the heating process is utilized as the hardening accelerator. According to this method, since there is no hardening accelerator when it is stored, in other words, any hardening acceleration function does not operate, it is possible to perform the refrigeration storage, thereby enabling the storage stability to be improved.

As the electrically conductive metallic powder, Sn, Ag, Bi, Cu, In, Ni, Sb, Pd or Pb itself or one or two species or more of metal or/and alloy in alloy consisting of the metals selected from the metallic powder group are exemplified.

As the organic acid, any generic organic acid may be used. Low molecular organic acid is preferable. In this embodiment, glutaric acid is used.

Executed Examples

The following will describe concrete examples in which the invention is applied to the electrically conductive joining agent in the executed examples. This invention, however, is not limited to the following concrete examples.

The following examination was performed to find out a combination of the electrically conductive metallic powder (solder) and the organic acid and the contents thereof when the organic acid metal salt was produced. All of the executed examples and the comparison examples contained electrically conductive metallic powder, epoxy resin, a hardening agent and an organic acid.

When the epoxy resin cures, any exothermic reaction is accompanied. Accordingly, the exothermic reaction was observed by a Differential Scanning calorimetry (DSC) and temperature when thermosetting occurred was measured. Measurement conditions of DSC were within a range of 25 through 300 degrees C. and temperature rising rate was 10 degrees C/min. The result thereof is shown in Table 1. The electrically conductive metallic powder used Sn, Cu or Ni and the organic acid used glutaric acid.

TABLE 1 Executed Comparison Comparison Example 1 Example 1 Example 2 Electrically Sn Cu Ni Conductive Metallic Powder Organic 2 2 2 Acid (wt %) Peak 204 255 280 Temperature (Degrees C.)

Sn an organic acid so as to be easy to produce organic acid metal salts and enables the epoxy resin to cure at lower temperature than that of other metal. As being also clearly shown from Table 1, it is said that Sn is preferable for curing at the lower temperature.

Table 2 showed how a period of time when the epoxy resin cured was influenced by a relationship of a molar ratio between the epoxy resin and the hardening agent and presence/absence of the organic acid metal salt. As the organic acid metal salt, glutaric acid-Sn salt was produced and it was executed using flux composition of the epoxy resin, the hardening agent and the organic acid metal salt. In the Table, a circle indicates curing; a triangle indicates semi-curing; and a cross indicates uncured.

TABLE 2 Executed Comparison Comparison Example 2 Example 3 Example 4 Molar ratio of Epoxy Resin: 1:2 1:2 No Hardening Agent Hardening Agent Organic Acid Metal Salt 2 — 2 Curing Curing Time: X X X Temperature: 3 min 150 degrees C. Curing Time: Δ X X 6 min Curing Time: Δ X X 9 min Curing Time: ◯ X X 12 min Curing Curing Time: ◯ X X Temperature: 3 min 220 degrees C. Curing Time: ◯ Δ X 20 min

As being clearly shown in the comparison example 4, it is understood that when there is no hardening agent even if the same epoxy resin is used, the curing does not advance. This is the same in a case where the curing temperature is 150 degrees C. or in a case where the curing temperature is 220 degrees C. below.

In the comparison example 3, a semi-curing was seen when about twenty minutes elapsed at a case of the curing temperature of 220 degrees C. while the epoxy resin and the hardening agent were mixed so that they have a molar ratio of 1:2. On the other hand, in the executed example 2, a semi-curing was seen when six minutes elapsed at a case of the curing temperature of 150 degrees C. below while the epoxy resin and the hardening agent were mixed so that they have a molar ratio of 1:2 and the organic acid metal salts of 2 weight % was further mixed to this mixture. It then completely cured when twelve minutes elapsed. Further, when setting the curing temperature to be 220 degrees C., it completely cured at only three minutes.

Thus, by adding the organic acid metal salts to the mixture of the epoxy resin and the hardening agent with a molar ratio of 1:2, it is possible to accelerate the curing and even when the curing temperature is set to be at a relatively low curing temperature of 220 degrees C., it is also possible to completely cure at around three minutes. This enables the curing to be performed for a short time.

Table 3 is a table showing overall characteristics of the electrically conductive joining agent according to the invention. The flux indicates the mixture containing the thermosetting resin, the hardening agent, the organic acid and a thixotropic agent. The thixotropic agent may be any conventionally used one such as hydrogenated castor oil, higher fatty acid amide and the like.

Further, a solvent of 0 through 20% may be added as needed and it may be volatilized for a reflow time. A composition of the solder is Sn-3.0Ag-0.5Cu. A composition rate of the organic acid, the thixotropic agent and the solvent in Table 3 is weight % in the flux composition and a composition rate of the flux and the solder is weight % in the electrically conductive joining agent. The peak temperature is a peak temperature by the exothermic reaction when the epoxy resin cures based on the DSC measurement.

TABLE 3 Executed Executed Executed Comparison Example 3 Example 4 Example 5 Example 5 Molar Ratio of 1:2 1:2 1:1 1:2 Epoxy Resin: Hardening Agent Organic 4 2 4 0 Acid Thixotropic Agent 6 6 6 6 Solvent 10 0 10 10 Flux: 12 12 12 12 the above Mixture Solder: 88 88 88 88 Sn—3.0Ag—0.5Cu Peak Temp 164 190 166 — (Degrees C.) Curing after 3 Cured Cured Cured Uncured minutes elapsed at 220 degrees C.

When comparing the executed example 3 with the executed example 4, the more the organic acid is added, an exothermic peak by thermal curing is shifted to a low temperature side. It is conceivable that the more the organic acid is added, many of the organic acid metal salts can be produced. Therefore, it is understood that a curing speed of the thermosetting resin can be accelerated by an amount of the organic acid.

It is understood from the comparison example 5 that the epoxy resin does not cure by heating at temperature of 220 degrees C. for three minutes if any organic acid is not included. It is also understood from the executed example 5 that when the molar ratio of the epoxy resin and the hardening agent is a mixture equivalent of mole, the epoxy resin cures but it is preferable to add the hardening agent more than the molar ratio of the epoxy resin because any unreacted epoxy resin remains by decrease of the hardening agent by means of the volatilization thereof or the like.

It is conceivable from which Sn is easy to produce the organic acid salts that when a ratio of Sn in the electrically conductive metallic powder is decreased, it is hard to produce the salts. Table 4 shows a result of test with the contents of Sn in the electrically conductive metallic powder being changed.

In the executed example 6, an alloy 1, Sn-3.0Ag-3.0Bi-3.0In, was used as the electrically conductive metallic powder. In the executed example 8, an alloy 2, Sn-3.4Ag-0.7Cu-2Bi-5Sb-0.04Ni, was used as the electrically conductive metallic powder. As other alloys, pieces of the electrically conductive metallic powder shown in Table 4 were used and shown in the executed examples 7, 9 and 10.

The peak temperature is a peak temperature by the exothermic reaction when the epoxy resin cures based on the DSC measurement. Reflow profiles in this moment are shown in FIGS. 1 through 4. FIG. 1 shows profiles of the executed examples 6 and 8; FIG. 2 shows a profile of the executed example 7; FIG. 3 shows a profile of the executed example 9 and FIG. 4 shows a profile of the executed example 10.

TABLE 4 Executed Executed Executed Executed Executed Example Example Example Example Example 6 7 8 9 10 Molar Ratio 1:2 1:2 1:2 1:2 1:2 of Epoxy Resin: Hardening Agent Organic 4 4 4 4 6 Acid Thixotropic 6 6 6 6 6 Agent Solvent 10 10 10 10 10 Ratio of 88:12 88:12 88:12 88:12 88:12 Solder: Flux Solder Alloy 1 Sn—10Sb Alloy 2 Sn—37Pb Sn—58Bi Composition Sn Contents 91 90 88.86 63 42 (%) Peak 160 158 155 169 158 Temperature (Degrees C.) Reflow FIG. 1 FIG. 2 FIG. 1 FIG. 3 FIG. 4 Profile Curing ◯ ◯ ◯ ◯ ◯ during Reflow

As being clearly shown from the executed examples 6 through 10, it is understood that even when the contents of Sn in the electrically conductive metallic powder are decreased, the organic acid metal salts can be produced. Even when the contents of Sn were 42% in the executed example 10, the epoxy resin could cure during the reflow process. Since a melting temperature range of Sn-58Bi in the executed example 10 is a range from 139 degrees C. to 141 degrees C., the peak temperature of the reflow profile is usually set to be higher than the liquidus temperature by 20 through 30 degrees C. Since the peak temperature by curing reaction in the epoxy resin is higher than the liquidus temperature of the solder but is lower than the peak temperature by the heating, the curing of epoxy resin occurs.

The invention contains the electrically conductive metallic powder including Sn of 40% or more, the thermosetting resin, the acid anhydride hardening agent, the organic acid and the thixotropic agent, wherein the electrically conductive metallic powder reacts with the organic acid during the heating process to produce the organic acid metal salts which is the hardening accelerator. Therefore, it is applicable to any electrically conductive joining agent containing the electrically conductive metallic powder including Sn of 40% or more, the thermosetting resin, the acid anhydride hardening agent and the organic acid, without limiting to the above-mentioned electrically conductive adhesive agents. Further, it is also applicable to any joining when any electrical conductivity is required other than an object to join various kinds of electronic circuits onto a circuit board.

INDUSTRIAL APPLICABILITY

This invention is applicable to the joining of the parts by the electrically conductive joining agent, the soldering by solder paste containing the thermosetting resin, a joint joined by the joining agent and the like. 

1. An electrically conductive joining agent containing: electrically conductive metallic powder including Sn of 40 weight % or more; thermosetting resin; an acid-anhydride-based hardening agent; and an organic acid, wherein the electrically conductive metallic powder and the organic acid are reacted to produce an organic acid metal salt and the organic acid metal salt is used as hardening accelerator.
 2. The electrically conductive joining agent according to claim 1, wherein the electrically conductive metallic powder includes Sn, Ag, Cu, In, Ni, Bi, Sb, Sb, Pd or Pb itself or one or two species or more of metal or/and alloy in alloy consisting of the metals selected from the metallic powder group.
 3. The electrically conductive joining agent according to claim 1, wherein the thermosetting resin is epoxy resin to which an aliphatic skeleton is given, the epoxy resin having flexibility.
 4. The electrically conductive joining agent according to claim 1, wherein as the acid-anhydride-based hardening agent, any of acetic acid anhydride, propionic anhydride, succinic anhydride and maleic anhydride is used.
 5. The electrically conductive joining agent according to claim 1, wherein the organic acid of 2 through 8 weight % is added
 6. A solder joint wherein the electrically conductive joining agent according to claim 1 is used. 